An absorbed current method has been used as a technique for specifying a failure portion of an interconnect in a semiconductor device. In the absorbed current method, an absorbed current amplifier (i.e., current detector) is connected to an interconnect narrowed down as being troubled, a region including the interconnect is irradiated with an electron beam, and a current image whose luminance is modulated according to the amount of the current flowing in the absorbed current amplifier is generated, thereby specifying a failure portion.
In this case, if the interconnect on a side of the detector with respect to a broken portion is irradiated with the electron beam, the current reaches the detector. In contrast, if the interconnect on a side opposite to the detector with respect to the broken portion is irradiated with the electron beam, the current cannot reach the detector. Therefore, if a difference between detected current amounts is modulated in term of luminance, and a current image is then generated by associating the difference with a position irradiated with the electron beam, a great difference in luminance is generated between the broken portion and an unbroken portion within the current image, and therefore the failure portion can be distinguished at a glance. The current image is generated, for example, by lighting a portion of a large current amount and darkening a portion of a small current amount.
An advantage of the absorbed current method resides in that the failure portion can be checked on the current image at a glance. This is because a portion to be originally interconnected does not appear in the current image in the case of an open trouble, whereas an interconnect not to originally appear in the current image appears in the current image in the case of a short trouble. However, in the case where the open trouble is not a complete open (i.e., a resistance≈∞) but a high resistance open (i.e., a resistance≠∞), there arises a problem that the detection of the failure portion becomes more difficult as the resistance becomes lower. The reason will be explained below.
In an equivalent circuit of a measurement system for the absorbed current method, an electron gun to generate an electron beam is represented by a constant current source having a predetermined output impedance Z. On the other hand, a high resistance open portion is represented by a predetermined resistance R. Further, moving a position irradiated with the electron beam on the interconnect corresponds to moving a connection point between the constant current source and the interconnect.
At this time, in the case where the relationship of Z>>R is established between the output impedance Z and the resistance R, there is no great difference in amount of a current flowing in the detector, irrespectively of whether the constant current source is connected onto the side of the detector with respect to the resistance R, or on the side opposite to the detector. In other words, as long as the relationship of Z>>R is established, a contrast on the current image is not varied near the high resistance open portion, or is very slight if varied. In this case, it is very difficult to recognize the high resistance open portion on the current image.
To overcome the difficulty, there have proposed some methods for detecting a slight change in resistance. In one method, one end of a suspected interconnect (i.e., interconnect suspected to be troubled) is grounded whereas the other end is connected to a detector. In another method, currents are drawn from both ends (starting point and ending point) of a suspected interconnect to differentially amplify the currents.
In the former method, the interconnect is grounded with low impedance, so that the influence of the output impedance of the constant current source need not be considered, and an absorbed current amount is changed according to the relationship between a ground resistance and a trouble resistance. Theoretically, as the ground resistance becomes smaller, this method can be more applied to a smaller trouble resistance.
In the latter method, a ratio of the currents drawn from the both ends depends on a ratio of a resistance generated between the starting point and the position irradiated with the electron beam to a resistance generated between the ending point and the position irradiated with the electron beam. Therefore, a change in resistance on the suspected interconnect can be detected by imaging the ratio of the currents.
Both of the methods are effective. However, in these methods, at least two points in the suspected interconnect need be probed for observation.
An interconnect in a semiconductor device is outputted from a gate and inputted into the gate. In other words, even if intermediate portions of the interconnect is located in an upper layer, the starting and ending points of the interconnect are ordinarily connected to input and output terminals of the gate located in a lowermost layer. Therefore, unless a failure portion in a suspected interconnect is previously found to be in an interconnect portion drawn on the upper layer, at least one portion ordinarily need be probed at the lowermost layer.
However, in a recent semiconductor device in which many interconnect layers are stacked, it is very difficult to connect a probe to an interconnect portion in a lowermost layer without damaging an interconnect portion in an upper layer, and therefore this is not practical. Should the probe be connected to two points in the lowermost layer, in the case where the interconnect is fanned out at plural portions with branches on the way, measurement need be repeated many times unless it is known in which branch a trouble is generated.
In conclusion, the technique for probing the suspected interconnect at two points is effective but not practical, and it is not impossible but difficult in practical use. Therefore, it is ideal in an inspection by the absorbed current method that measurement can be performed by the connection to the suspected interconnect in the upper layer at one point. In addition, it is desirable that the measurement by the connection at one point should enable the high resistance open trouble to be detected.
JP-A 2008-270632 (KOKAI) discloses an inspection apparatus for scanning with a charged beam while vibrating a position irradiated with the charged beam. On the other hand, JP-A 2008-211111 (KOKAI) discloses a sample inspection apparatus having a plurality of probes.